The invention relates to a device for determining the axial position of hollow cylinders. The determination and definition of the axes of hollow cylinders represent a certain problem, since the hollow cylinders to be measured have only roughly an ideal shape. Depending on the product and the product quality, the cross section of hollow cylinders over the axial length is only roughly circular and thus of variable radius. Still, it is possible to reach the center point for a stipulated cross section using a computation method to be stipulated, for example determining the center of gravity over the cross sectional surface. The center points are connected along the axis of a hollow cylinder by a line which however is exactly straight only in the ideal case. For precision applications, in practice a connecting line which is curved in space can be expected. It is often desirable to determine the center axis of hollow cylinders not only within or on the ends thereof, but also to extrapolate beyond its ends in order to be able to position succeeding machine parts in an exact manner. This is especially desirable when requirements according to a standard must be met, as for example standard DIN/ISO 9000. These precision applications can be for example: Precision tubing in the chemical industry or in nuclear power plants, shaft jacketing in shipbuilding, or production of gun barrels, etc.
The object, i.e. to devise one such measurement process which allows the recording of the axial position of the hollow cylinder even at some distance, is achieved with the features which are given in the independent claims.
The invention is based on the fact that inaccuracies in the position of a laser beam relative to its housing are not regarded as unwanted errors, but in contrast can be used as the basis of the measurement process as claimed in the invention. The beam position of a laser, for example a semiconductor laser, can typically be adjusted only with an effort to be exactly symmetrical to its housing dimensions. Normally a lateral offset (for example radial offset) and an angular offset with reference to the axis of symmetry of the housing can be expected. This results in that the location of a laser beam is typically askew to an ideal axis.
The invention is based on the finding that when one such askew axis is turned around an ideal axis the surface of a hyperboloid is defined. A target plane located perpendicular to the ideal axis is therefore cut in an exactly circular manner by the clamped hyperboloid surface. As claimed in the invention it is provided that a suitable device be prepared with which a laser beam which has been set randomly or intentionally askew can be turned relative to the ideal axis of a hollow cylinder and within the hollow cylinder. At some distance outside the hollow cylinder, in this way the laser beam on a target surface can produce circular lines which depending on the distance of the target from the hollow cylinder have different diameters. The magnitude of these diameters depends on to what extent the askew offset of the laser beam axis from the ideal axis of the hollow cylinder to be measured is present. To make available a suitable device, with which the required rotation, optionally the adjustment of different askew values of the laser beam can be carried out, the invention calls for rollers which can roll on the inner jacket of a hollow cylinder to be measured and for purposes of precise support with a clamping device can be placed against its inside diameter. Preferably the clamping device is made similar to joints of an umbrella, but can also be implemented by clamping cones or comparable structures. Advantageously as claimed in the invention there are two clamping devices which can be adjusted independently of one another and which are otherwise structurally roughly identical or interchangeable. To produce a light beam, especially a laser light beam, a battery-operated semiconductor laser is advantageously used. The laser light source can be turned by hand or using a motor. For motorized drive, in addition to electricity, also compressed air can be provided, for example in the form of a slow-speed turbine engine. Especially economical targets are commercial air rifle targets which are used once and facilitate the marking of an ideal position with one prick or the like on a reference surface. For more precise measurements it can be provided that the middle point position of a circular ring projected by a laser beam be determined with optoelectronic means, for example using a so-called CCD sensor, in interaction with a computer.